1. Field of the Invention
The present invention relates to an image-reading device which has a photoelectric transducer designed to input image data.
2. Description of the Related Art
Image-reading devices each having a photoelectric transducer are used in facsimile machines, copying machines, image-recognizing apparatuses, optical filing systems, and the like. Recently it has been increasingly demanded that the image-reading device of this type operate at higher speed and output data representing a high-quality image. Hence, it is now required that the image-reading device comprise a photoelectric transducer which is highly sensitive and can convert optical signals to electric signals at high speed, and also that the electric signals output by the transducer be processed with high accuracy within the image-reading device.
Various photoelectric transducers are available at present. Of these, the CCD line sensor is the most popular. In an image-reading device having a CCD line sensor and an image-reading unit, the signals output by the sensor are subjected to shading correction, bit by bit, thereby providing image data. The shading correction comprises two steps. In the first step, the one-bit pixel data items defining an image are corrected based on the difference between the black reference signal and the white reference signal, thereby forming corrected image data. In the second step, the low-frequency distortion and high-frequency distortion occurring in the data-reading unit are minimized in accordance with the corrected image data. (Such shading correction is disclosed in Published Unexamined Japanese Patent Application No. 62-293384.)
Shading correction will be described in more detail, with reference to the graph of FIG. 1 which shows the results of the shading correction performed in a method.
As can be understood from FIG. 1, a signal VoutO output by a CCD line sensor which is reading an image formed on an original. The CCD line sensor outputs a black reference signal V.sub.BK upon receipt of the light reflected from an original having reflectance of 0%. The magnitude of the signal which the CCD line sensor generates from the light reflected from an original is linearly proportional to the reflectance of the original. When the CCD line sensor receives the light reflected from an original having reflectance of 100%, it outputs a white reference signal V.sub.WT -V.sub.BK. It should be noted that the term "reflectance of 100%" means the reflectance of a standard white board, not actual "100%."
In FIG. 1, Vout1 is a signal which has been obtained by converting an output signal of the CCD line sensor to an 8-bit digital signal, and then by subjecting the digital signal to normalization, wherein the black reference signal V.sub.BK and the white reference signal V.sub.WT -V.sub.BK having values of OO(H) and FF(H), respectively are applied. Here, the following relation is established: EQU Vout 1=FF.times.(VoutO-V.sub.BK)/(V.sub.WT -V.sub.BK)
The normalization, thus effected, minimizes both high-frequency distortion and low-frequency distortion, enabling the output signal of the CCD line sensor to change linearly from OOH to FFH.
The higher the sensitivity of the CCD line sensor, the greater the possibility that an amplifier is connected to the output of the CCD line sensor. The characteristic of the amplifier affects that of the CCD line sensor. In the worst case, the sensor fails to output whose magnitude changes linearly, in accordance with the intensity of the input light. Consequently, if the shading correction is effected on the output signal of the CCD line sensor based on only the black and white reference signals of the types described above, the output signal of the CCD line sensor will deviate, in magnitude, from the most desirable output signal, as is illustrated in FIG. 2.
As the sensitivity of CCD line sensors increases at the expense of the linearity of input-output characteristic, the conventional shading correction, in which use is made of only the black reference signal and the white reference signal, can no longer render the output signal of a CCD line sensor exactly proportional to the intensity of the light applied to the sensor.